Question about restarting water heaters

[Tab]

I have oil-based heating, including my hot water heater. We recently ran out and so I temporarily put in some diesel and restarted the furnace. So, my question is to the most efficient way to get the hot water heater heated up. I could leave it as is, and it'll have to reheat the whole tank (I think it's 40 gallons). Or I could draw down the water from the tank, forcing it to refill, but it should be more efficient since it has less to heat to get it up to temp.

Thoughts?

[Warpy]

1. it's going to have to heat that 40 gallons no matter what

and the only difference I can see in starting with an empty tank is speed on those first few gallons. If you are absolutely desperate for a hot shower and don't mind running out in the middle of it, finishing in cold water, it might be a plan.

However, you are going to use the same amount of energy to heat 40 gallons of water, whether you do it all at once or start with a dry tank.

The best way to save money is to turn down the temperature in the summer, when you're not using the hot water for heating.

[Tab]

2. You don't think

(just theoretical at this point)

Heating 10 gallons in water, and heating subsequent water, beats trying to do all 40 g at once?

(It's in the process now, so it's really a theoretical discussion).

[Thor_MN]

3. Not familiar with any type of water heater that doesn't heat a full tank.

In fact, starting a tank based heater without the tank full could damage the heater. Just start it back up.